(1) Field of the Invention
The present invention relates to a separator for polymer electrolyte fuel cells and processes for production thereof.
(2) Description of the Prior Art
Fuel cells have excellent features such as (1) it is virtually unnecessary to use any fossil fuel which may be exhausted in a not too distant future, (2) substantially no noise is produced in electricity generation, and (3) energy recovery is high as compared with the cases of other methods of electricity generation. Therefore, utilization of fuel cells in relatively small power plants in building or factories is being studied.
Of fuel cells, polymer electrolyte fuel cells operate at low temperatures and have no problem of corrosion of cell parts, as compared with other types of fuel cells and, moreover, can generate a relatively large electric current at low operating temperatures. Therefore, polymer electrolyte fuel cells are drawing attention as a substitute for internal combustion engines of automobiles.
In polymer electrolyte fuel cells, the separator used therein as one component has roles of (a) providing a passage of a reactive gas fed into the fuel cell the (b) transmitting the electricity generated in the fuel cell, to outside, and (3) dissipating the heat generated in the fuel cell. In order to perform these roles, the separator must satisfy requirements of being light in weight, high gas barrier property and easy cuttability for groove formation.
The separator used in polymer electrolyte fuel cells has heretofore been made of graphite impregnated with a resin (e.g. phenolic resin) or graphite having a glassy carbon layer formed thereon, in view of processability and cost.
The graphite impregnated with a resin is expensive because a step of impregnation and drying must be repeated a plurality of times in order to allow said graphite to have a desired gas barrier property. Further said graphite has a high density because of the high density of graphite and makes the total weight of the fuel cell large.
The graphite having a glassy carbon layer formed thereon requires a complicated process and is expensive because a step of impregnation and drying is repeated a plurality of times as in the case of the graphite impregnated with a resin and then is fired in a non-oxidizing atmosphere. Further said graphite has a high density because of the high density of graphite and makes the total weight of the fuel cell large.
It is considered to use glassy carbon as a material for a separator, as is done in phosphoric acid fuel cells. In this case, glassy carbon is lighter than graphite and the total weight of the fuel cell is small. However, glassy carbon is expensive and, moreover, fragile, making the groove formation therein very difficult and requiring a high processing cost.
In order to solve the above-mentioned problems of the separators for fuel cells, the following proposals were made. That is, there was proposed a method which comprises mixing a thermosetting resin and a carbon material, molding the mixture into a separator shape, and firing the molded material in an atmosphere of inert gas or the like at a high temperature to carbonize and graphitize the thermosetting resin (U.S. Pat. No. 4,855,092, JP-A-59-154770, JP A-60-90807 and JP-A-62-59508). This separator obtained by firing and carbonization, however, has problems in that the carbonization incurs a high cost and, moreover, owing to its complicated structure (such a complicated structure is ordinarily possessed by separators for fuel cells, for exhibition of high cell performance), cracking, uneven contraction, etc. occur during firing, making the produced separator unusable as a separator for a fuel cell which must have high dimensional accuracy.
In order to solve the above problems, there was also proposed a process which comprises mixing a carbon material and a resin, molding the mixture into a separator shape, and using the molded material itself as a separator. This method includes, for example, a method which comprises mixing a conductive agent which is a mixture of expanded graphite particles and other carbon particles (spherical, bulky or carbon fiber), to a resin (JP-A-1-311570); and a method which comprises mixing expanded graphite with a water-repellent substance and molding the mixture under pressure (JP-A-1-154467). In the former method described in JP-A-1-311570, the separator obtained is inexpensive and has dimensional stability. However, there is a problem in that the mixing of expanded graphite with other carbon particles does not proceed well because the expanded graphite is inherently low in bulk density, inviting reductions in gas barrier property and strength (these are the essential properties to be possessed by separators for fuel cells). In the method described in JP-A-154467, the separator obtained is unusable in a fuel cell of low cost because the water-repellent substance (a resin of high water repellency or a water-repellent powder) used is expensive; the high water repellency of the water-repellent substance makes bad its miscibility with carbon, allowing the resulting separator to have a low strength; and the high water repellency of the separator obtained allows the water generated during power generation to block the water path easily, resulting in reduced cell efficiency.
It is also considered to use a carbon composite material of expanded graphite and a resin, proposed for an electro-magnetic shield in JP-A-3-181532. However, the carbon composite material mentioned in JP-A-3-181532 is high in resin content, has a very high electrical resistance, and is unusable as a separator for fuel cell.
Therefore, it has been necessary to develop a separator for polymer electrolyte fuel cells, which is lightweight, can be grooved precisely and easily, and has a high gas barrier property, strength and electroconductivity.